Abstract: A squall line attacks Jiangxi, Zhejiang and Fujian provinces on April, 12 2003. The severe convective system leads to heavy damage, producing wind gust as large as 32 m/s, 30 mm diameter large hails in the affected area. This process is observed by the CINRAD in Jianyang City. The whole process of this convective system, including the initial stage, the developing stage, the mature stage and the dissipating stage, is analyzed. Many convective structures are observed in the process. At the location of the strong updraft, base reflectivity observed by mid-level elevation is stronger than the low-level and the high-level. This structure is called weak echo area. Corresponding to weak echo, there is a positive velocity area within a large negative velocity area in the mid-level. This structure is judged as mesoscale vortex sometimes by WSR-98D PUP. In fact, this phenomenon means the convergence in the target area, which is known as MARC (mid-level radial convergence). Strong line echoes are observed in squall line by radar. It is well known that bow echoes that develop within a squall line are referred to as line echo wave patterns. In order to analyze the interior structure of the convective system, mesoscale numerical model (MM5V3) is used to simulate this process. In the control numerical simulation, reanalysis NCEP data (1°×1°) is used as the initial conditions. Radar observation data and simulation result are used to analyze the structure and the evolvement. Numerical simulation result confirms that the convective system takes place at the meeting of the northeaster and the southwester. There are many convective vortices and super cells within the 400 km system. Bow echoes appear near the vortices. There are obvious cyclones at the forehead of the line echo and divergence at the tail. Weisman indicates that in a mature bow echo cyclonic and anticyclonic vortices develop north and south respectively, in the channel of rear-to-front flow. These results resemble Weisman's study about squall lines in American. In the period from 0758UTC to 0840UTC, there are respective bow echoes in the north and west, seen from the reflectivity PPI. These results confirm that there are segments of mesoscale vortices and bow echoes in squall lines. Jianyang city is to the east of Wuyi Mountain, and there is a bell-mouthed mountain to the west of Jianyang city, so the influence of the topography should be considered. In order to do that, a sensitive numerical simulation is designed. In the sensitive numerical simulation, the altitude of the mountains near Jianyang is factitiously set equal to the ground. The results of the control and sensitive numerical simulation are compared. It shows that the uplifted velocity in 500 hPa level of control simulation is greater than the velocity of sensitive simulation, and the uplifted area accords with the strong reflectivity echo. These results refer that the windward mountain and bell-mouthed mountain can trigger new convective cells or enhance existing ones.